1. Field of the Invention
The present invention relates to surface modified-colour pigments and methods for preparing them. The present invention also relates to non-aqueous pigment dispersions made with these pigments.
2. Description of the Related Art
Polymeric dispersants are substances for promoting the formation and stabilization of a dispersion of pigment particles in a dispersion medium. They typically contain in one part of the molecule so-called anchor groups, which adsorb onto the pigments to be dispersed. In a spatially separate part of the molecule, polymeric dispersants have polymer chains compatible with the dispersion medium, thus stabilizing the pigment particles in the dispersion medium.
In aqueous pigment dispersions, the polymeric dispersants generally contain hydrophobic anchor groups exhibiting a high affinity for the pigment surface and hydrophilic polymer chains for stabilizing the pigments in the aqueous dispersion medium.
Thermally stable non-aqueous dispersions with submicron particles are much more difficult to prepare, especially for pigments having a non-polar surface. Particularly high requirements are set when manufacturing solvent based, oil based and radiation curable inkjet inks. Inadequate dispersing manifests itself as increased viscosity in liquid systems, loss of brilliance and/or hue shifts. Moreover, particularly good dispersion of the pigment particles is required to ensure unimpeded passage of pigment particles through the nozzles of a print head, which are usually only a few micrometers in diameter. In addition, pigment particle agglomeration and the associated blockage of the printer nozzles has to be avoided in the standby periods of the printer.
One approach to disperse pigments with a hydrophobic surface in a non-aqueous dispersion medium is by modifying the pigment surface to a hydrophilic surface by addition of so-called dispersion synergists. WO 2007/060254 (AGFA GRAPHICS) discloses quinacridone derivatives having carboxylic acid groups for modifying the surface of quinacridone pigment so that non-aqueous pigment dispersions exhibiting high dispersion quality can be prepared. Sulfonated quinacridone synergists are disclosed as comparative examples (SYN-1 to SYN-3), but however fail to improve the dispersion stability of quinacridone pigments.
Sulfonated quinacridone derivatives have also been disclosed in, for example U.S. Pat. No. 6,152,968 (BASF), which is however silent on their use in non-aqueous media.
In another approach, the pigment surface is modified by direct chemical reaction. WO 2008/034472 (AGFA GRAPHICS) discloses a surface modification of several yellow pigments by hydrolyzing their methyl ester groups though the addition of sulphuric acid.
US 2003134938 (DAINICHISEIKA) discloses a process producing a sulfonated solid particle by burning sulphur to yield gaseous sulphur dioxide, subjecting the gaseous sulphur dioxide to catalytic oxidation to yield gaseous sulphur trioxide, and sulfonating a dry powdery or granular solid particle with the gaseous sulphur trioxide in a gas phase-solid phase reaction. The pigments in Table 1 include the quinacridone pigment C.I. Pigment Violet 19. Sulphur dioxide and sulphur trioxide are highly toxic compounds which can also cause chemical burn on the skin of operators. From viewpoints of workability and safety, their use is to be avoided. Also from an environmental perspective sulphur dioxide is to be avoided since it results in acid rain damaging forests and crops.
For consistent image quality, inkjet inks require a dispersion stability capable of dealing with high temperatures (above 60° C.) during transport of the ink to a customer, jetting at elevated temperatures and changes in the dispersion medium of the inkjet ink during use, for example, evaporation of solvent and increasing concentrations of humectants, penetrants and other additives.
Therefore, it is highly desirable to manufacture in a cost effective way non-aqueous pigment dispersions exhibiting high dispersion quality and stability.